With the passage of time the volume and variety of solid waste products requiring disposal continually increases. In the past it was common practice to burn such waste products in open incinerators. However, because of comprehensive environmental regulation, incineration of solid waste has been restricted to a significant extent in many geographic areas and is prohibited in most urban areas.
Disposal of solid waste products in sanitary landfills has heretofore been a frequently used alternative disposal method. Presently, however, many existing landfills are reaching their capacity and additional clean landfills have not been approved by federal, state and local regulatory agencies due in part to existing environmental regulations and also because shortages of land in some geographic areas. Reemphasis has thus been placed upon incineration as a principal method of solid waste disposal. With that has come cleaner burning incinerators and a desire to recover, to the fullest extent possible, metals and other recyclable by-products of the incineration process.
Solid waste incineration produces large quantities of ash as a by-product. Although highly friable, ash possesses considerable adhesive and cohesive qualities. Consequently, ash tends to cling to the surface of ferrous metals and other salvageable materials and must be physically separated therefrom before those materials can be effectively recycled. Some techniques for ash removal have been rather crude, whereas others have been unduly complicated.
For instance, it has long been known to dislodge residual ash from the surface of salvageable waste products by percussive force. At its simplest, this is achieved by scooping a quantity of incinerated matter using the shovel or an electromagnet of a crane, bulldozer, front end loader or similar apparatus, raising the shovel or electromagnet to the desired elevation and then dropping the matter onto a hard surface. Many cycles of lifting and dropping are usually required before the recoverable materials are sufficiently "clean" to be recyclable. Additionally, this method is quite messy, as well as time and labor intensive.
It has also been proposed to drop quantities of incineration by-products against an inclined screen, grate or the like. In so doing, friable material such as ash is caused to fall through the openings in the screen and salvageable material of a predetermined particle size is caused to roll off the screen, whereby it may be recovered. This method, although somewhat more sophisticated, suffers from the same disadvantages encountered when simply dropping matter against a hard surface. Moreover, screens quickly tend to clog thereby reducing their effectiveness. Conversely, even an unclogged screen performing at optimum efficiency is undesirable because a considerable fraction of valuable salvageable material falls through the screen's openings and is unintentionally discarded.
Others have used trommels, which are simply rotating cylindrical screens, as means to extract ash from salvageable matter. Some trommels are internally equipped with means for agitating the waste material during trommel rotation. Being screens, however, trommels clog and also waste salvageable material. An example of a trommel separator is disclosed in U.S. Pat. No. 4,020,992.
U.S. Pat. Nos. 3,086,718, 3,650,396, 3,885,744, 3,973,736, 4,044,956, 4,341,353 and 4,815,667, as well as published European Patent Specification 0 220 853 describe recovery systems including mill-type crushing and grinding devices for comminuting the waste products to reduce the size of the products, oftentimes both the salvageable and non-salvageable materials, as well as several stations at which the comminuted products are separated by size. Although generally effective for their intended purposes, such apparatus require frequent maintenance. In particular, they encounter little resistance in crushing malleable materials including non-ferrous metals such as aluminum and the like and friable materials such as ash. However, hard, crush-resistant materials such as ferrous metals tend to cause frequent jams and premature wear of the grinding elements. Moreover, consumers of recovered ferrous metals, typically steel mills, are generally not concerned that the raw ferrous metals be reduced to small and/or uniformly sized fragments. Little need exists, therefore, to comminute ferrous metal products or to separate such products according to size. The most important consideration is that the ferrous metal products that are recovered be "clean," i.e., essentially uncontaminated by nonmetallic waste products and ash.
U.S. Pat. No. 4,337,900 describes a system for recovering aluminum from unincinerated waste. The system includes a preliminary magnetic separator and a final magnetic separator. The preliminary magnetic separator removes a certain fraction of ferrous metals from a stream of waste material but is not supplemented by any means for removing matter that may cling to the surface of the ferrous metals. As such, ferrous metals recovered by the preliminary magnetic separator are generally not sufficiently clean to enable an end user to recycle the ferrous materials without first performing additional time, labor and energy consuming procedures to separate the residual waste matter from the ferrous metals. The final magnetic separator follows a hammer ring rotor where ferrous metal and other components of the waste stream are comminuted. As with the systems discussed immediately hereabove, such comminution of the hard ferrous metal components tends to shorten the useful service life of the comminuting equipment.
U.S. Pat. No. 3,802,631 discloses a raw refuse material separating and recycling system that employs a complex arrangement of material separation stations. The first of these stations employs a slurry drum separator. The slurry drum is a rapidly rotatable trommel connected to a supply of water whereby water is injected into the drum to form a slurry of refuse which, during rotation of the drum, is forced outwardly by centrifugal force to pass through one or more screens in the annular side wall of the trommel. The drum separator thus initially separates the refuse into soluble and insoluble materials. The slurry drum separator consumes considerable quantities water in order in order to separate the soluble from the insoluble matter. In addition, it does not separate ferrous materials from the balance of the refuse stream. This occurs at a magnetic separator located several stations downstream of the slurry drum. If conceivably adapted to an incinerated waste products salvaging environment, the apparatus disclosed in U.S. Pat. No. 3,802,631 would be unnecessarily complex and resource intensive. Moreover, it would constitute an inefficient and unduly expensive system for one seeking an economical way to separate ferrous metals from other incinerated waste by-products.
A need exists, therefore, for an uncomplicated system that will permit cost-effective recovery of ferrous metal components from friable, incinerated waste materials including carbonaceous incineration by-products such as ash. Such a system should enable efficient recovery of clean salvageable ferrous metal components from the incinerated waste materials without comminution of the ferrous metal components and without using supplemental resources such as water as a separating agent.